PHOENIX – Investigators searching a stretch of the U.S.-Mexico border for clues into the fatal shooting of a Border Patrol agent face a treacherous territory that is heavily used by drug smugglers, offers many hiding places and is close enough to Mexico for traffickers to make a quick getaway.

Whoever killed Agent Nicholas Ivie and wounded another agent in the sparsely populated desert in southeastern Arizona early Tuesday may have done just that.

Those who carried out the shooting near Bisbee, Ariz., probably had time to cross the border in the early-morning darkness before authorities could seal off an escape route, said George McCubbin, president of the National Border Patrol Council, a union representing about 17,000 border patrol agents.

"I seriously doubt anybody would be laid up and hiding," he said.

Ivie and two other agents were fired upon in a rugged hilly area about five miles north of the border as they responded to an alarm that was triggered on one of the sensors that the government has installed along the border. The wounded agent was shot in the ankle and buttocks and released from the hospital after undergoing surgery. The third agent wasn't injured.

Ivie was a 30-year-old father of two who grew up in Utah and was active in the Mormon church. He was an agent for four years.

Authorities have declined to provide other details, including what they believe prompted the shooting and whether the agents were ambushed. Still, they suspect that more than one person fired on the agents. No arrests have been made.

The last Border Patrol agent fatally shot on duty was Brian Terry, who died in a shootout with bandits near the border in December 2010. Terry's shooting was later linked to the government's "Fast and Furious" gun-smuggling operation, which allowed people suspected of illegally buying guns for others to walk away from gun shops with weapons, rather than be arrested.

Authorities intended to track the guns into Mexico. Two rifles found at the scene of Terry's shooting were bought by a member of the gun-smuggling ring being investigated. Critics of the operation say any shooting along the border now raises the specter that those illegal weapons are still being used in border violence.

A federal law enforcement official who spoke on the condition of anonymity told The Associated Press that no weapons have been found. The official requested anonymity because information on the search hasn't been publicly released.

The shooting occurred in an area heavily frequented by drug smugglers, though less so in recent years by illegal immigrants crossing the border, said Dave Stoddard, a retired Border Patrol agent who worked in the agency's station in the area for eight years. "You're talking about cocaine alley," Stoddard said.

The area historically has been popular with smugglers because U.S. 80 comes within several miles of the border, allowing backpack-toting smugglers to take loads of drugs to the highway for pickup by vehicles. "It takes maybe less than a minute and everybody leaves the scene," Stoddard said, explaining that the "mules" either get in the vehicle or go back into the desert to return to Mexico on foot.

The desert where the shooting occurred is dotted with creosote and other brushes that hamper visibility at ground level. Gullies and ridges also provide cover for smugglers.

The area is part of the nation's busiest Border Patrol sector, which received additional agents and fencing as the federal government sought to improve border security in recent years.

To sneak drugs into the country, teams of smugglers who lug up to 40 pounds of marijuana each in backpacks made of burlap and flour bags typically have to either walk over the border or scale the fence that covers about 30 percent of the 2,000-mile U.S.-Mexico border.

Agents patrol the fence by driving along roads beside the barriers and monitor the area through surveillance cameras mounted on nearby towers and sensors placed along smuggling routes after the fence.

Agents who may find it hard to spot smugglers in the brush use remote sensors to detect illegal border-crossers so agents can be deployed in response. Seismic sensors buried in the ground on paths and trails in the desert and mountains north of the border are used to detect the passage of people, animals and vehicles.

Each Board Patrol station monitors hundreds of such sensors and can dispatch agents to the scene when the devices are triggered. "As people walk over that, that will trigger the signal," McCubbin said. "You go out and check it out."

Multiple sensors can be deployed along a path so agents can tell the direction of travel and get an idea of how large a group is involved, he said. Then agents can either get ahead of the group or come in from behind and use night-vision goggle sand infrared cameras to spot them.

Border Patrol agents often are posted several hundred feet and even miles away from the border to look for people who sneaked into the country. Once smugglers get past several layers of enforcement, they typically hook up with a driver on a highway or ranch road and use back roads to make their way around Border Patrol checkpoints.

In the county where the shooting occurred, checkpoints could be seen on two highways that carry traffic to and from Interstate 10.

Drug-sniffing dogs and agents screen traffic passing through the checkpoints, resulting in searches of vehicles if there are indications of smuggling activity. But the checkpoints' real enforcement purpose is indirect, Stoddard said.

"The real purpose of the checkpoint is to get the dope or people on foot in a remote area where they can be picked up. It's gravy when a load goes into the (checkpoint)."